1. Field of the Invention
The present invention generally relates to a calibration apparatus used for a multi-element sensor. More specifically, the present invention is directed to such a calibration apparatus capable of calibrating a high-resolution multi-element sensor used to sense an optical image of a facsimile, or an earth-monitoring multi-element sensor installed on a satellite.
2. Description of the Related Art
Conventionally, a multi-element sensor installed in a facsimile, or a satellite may sense an image signal, and further is electrically calibrated so as to periodically confirm operation conditions of electronic circuitry employed in an entire system.
One typical multi-element sensor introduces the following electric calibration method. That is, while using two levels of signals, namely, a zero level signal and an electric calibration signal as a reference, the sensor signal is electrically calibrated.
FIG. 7 is a timing chart for the various signals related to the typical multi-element sensor with respect to the time sequential operation.
As shown in FIG. 7(a), an image signal is outputted from the multi-element sensor during a time period of "t1", either a zero level signal or an offset signal is outputted during a time period of "t2", and an electric calibration signal is outputted during a time period of "t3". The time period of "t2" succeeds the first-mentioned time period of "t1", and the time period of "t3" succeeds this time period of "t2".
Conventionally, the image signal outputted during the time period of "t1" is calibrated based upon both the zero level signal outputted during the time period of "t2" and the electric calibration signal outputted during the time period of "t3".
FIG. 7(b) is a timing chart for indicating an image signal which has been calibrated by employing the zero level signal and the electric calibration signal with respect to the time sequential operation.
This conventional calibration method is described in, for example, Japanese Patent Laid-open Publication Nos. Hei5-183143 and Hei9-37022.
On the other hand, in a multi-element sensor, a minus-offset may be set to a zero level signal in order to widen a dynamic range of this multi-element sensor. Also, a zero level of this multi-element sensor may be set and this set level exceeds a preselected sensitivity, since this sensor may detect a light amount higher than a preselected light level.
In this conventional sensor calibration technique, since the image signal is calibrated based upon the two levels of the zero level signal and the electric calibration signal, both the offset and the gain of this sensor can be hardly corrected.
Also, the conventional multi-element sensor calibration apparatus presents the following problem. That is, within the image signals outputted from the multi-element sensor whose elements are arranged in a linear manner, the image signal outputted from the even-numbered multi-element sensor and also the image signal outputted from the odd-numbered multi-element sensor pass through the different registers and the different amplifiers. Since the offset amounts of the respective registers and also the gains of the respective amplifiers are different from each other, there is a slight difference between the electrical characteristics of the image signal derived from the even-numbered multi-element sensor, and the electrical characteristics of the image signal derived from the odd-numbered multi-element sensor.